U.S. Homeland Security Secretary Jeh Johnson says his department is reviewing a program allowing foreigners to travel to the United States in order to marry U.S. citizens following last week's mass shooting in San Bernardino, California.

Johnson said the review was being carried out after it was revealed that Tashfeen Malik, the Pakistani woman accused of carrying out the massacre with her husband, was one of 519 Pakistanis approved to enter the United States in 2014 to marry a U.S. citizen.

Malik's path to the United States immediately raised questions about the government's system for vetting foreign fiances -- the so-called K-1 visa program.

State Department records show that out of more than 9.9 million visas issued during 2014, about 36,000 were fiance visas under the K-1 program.

Malik and her husband, Syed Rizwan Farook, are accused of killing 14 people at a social center in California before they were killed in a gunbattle with police.